The influence on selectivity of the aluminum content in the matrix of FCC catalysts

The influence that both the type of matrix and the interaction between zeolite and matrix have on the selectivity of FCC catalysts was studied by means of the conversion of cyclohexene at 300degreesC on a large number of samples in which the matrix was changed. Silica/alumina matrices had 0, 12 and 25% of alumina, and catalysts were subjected to steaming of varying severity followed by acid extraction in some samples to remove extraframework aluminum species (EFAI). Resulting catalysts were characterized by various techniques. It was confirmed that hydrogen transfer does not depend directly on the type of coordination of the aluminum atoms in the sample, but rather on the density of paired sites in the zeolite component. It was possible to define the selectivity of reaction pathways (SAP) as an index to describe the relative importance of the processes of desorption via hydride transfer to yield cyclohexane against retention of the cation cyclohexil via isomerization and further reaction. A high value of S(RP) would mean that a given catalyst has a lower ability to retain adsorbed species that can be subjected to additional reactions like, in this particular reaction, isomerization and further proton transfer to the catalyst surface, or cracking. The index was shown to increase whereas the relative amount of octahedral aluminum atoms decreased, a fact that can be associated to the formation of a new silica/alumina phase. Such phase would be formed by means of the reaction of aluminum extracted from the zeolite upon steaming and silica present in the matrix. (C) 2002 Elsevier Science B.V. All rights reserved.